The question of what to do when something is not detected, but the
detection limit is not zero, is common in chemistry, and also has legal
ramifications for industries, utilities, cities, etc. complying with EPA
pollution limits etc.
In the legal situations the answer is clear... the "below detection
limits" must count as zero.
I can give you many examples that I have been involved with. Here is
one...
A refinery discharges process water into a river. The process water
goes through a treatment facility prior to arriving at the outfall into
the river. The treatment facility is required to test for all sorts of
things such as pH, biological oxygen demand (BOD), and various chemicals
known to appear sometimes or on a regular basis.
An element present in crude oil is selenium (Se). Depending on the
particular crude being processed, the Se might be high or low. The
permit states that Se discharge to the river shall never exceed 10
parts-per-million (ppm). In addition, the Se level shall be tested
twice a day, once in the morning and once in the afternoon, and the
10-day rolling average shall not exceed 4 ppm. The accepted detection
limit of the monitoring process is 1 ppm.
As a matter of legal definition, any time the test comes out zero (or
actually any reading less than 1 ppm) the number is recorded as "not
detected" AND the number used in the 10-day rolling average is zero.
It pretty much has to be this way. If the water going to the outfall
had one reading of 5 ppm, followed by 9 readings of "not detected," and
the not-detected readings were simply "thrown out," then the 10-day
rolling average would be 5 ppm (the single reading above the detection
limit) and the refinery would have a permit violation. That is clearly
wrong. The refinery is definitely below the 4 ppm 10-day average. You
can not throw these values out.
On the other hand, if the refinery had five readings of "not detected"
and five readings of 8 ppm, and if the not detected values were counted
as 1 ppm, then the five 8 ppm readings and the five 1 ppm readings
(really not detected) would average to 4.5 ppm, which is also a
violation. Yet, it is not provable that the refinery did exceed the 4.0
average (nor is it provable they didn't).
You can't cite a violation and issue a fine when there is no evidence
that the company was actually in violation. The "not detected" readings
have to be averaged as zeroes.
I don't know what happens outside of environmental permit compliance,
but I have worked on a liaison committee between the refinery, the
health department, and the Ohio EPA for the past 20 years, and it is
clear that for average values calculated for legal permit compliance
purposes, the "not detected" values are averaged as zeroes. I don't see
how it could be any other way.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu